Furnace organization



2 Sheets-Sheet 1 Filed July 25, 1957 Gas Fig. l.

(MR A4 062a ATTORNEY Oct. 10, 1961 P. c. EUCHNER, JR

FURNACE ORGANIZATION 2 Sheets-Sheet 2 Filed July 25, 1957 Fig. 2.

INVENTOR Perry G. Euchner, Jr.

ATTORNEY United Stat of 3,003,441 FURNACE ORGANIZATION Perry C. Euchner,Jr., Ridgewood, N.J., assignor to Combustion Engineering, Inc., NewYork, N.Y., a corporation of Delaware Filed July 23, 1957, Ser. No.673,677 7 Claims. (Cl. 110-49) This invention relates generally tofurnaces wherein hot combustion gases are recirculated back to thefurnace and has particular relation to an improved duct and system forreintroducing these gases intothe furnace.

As is known in steam generating practice, superheated steam temperaturemay be controlled by gas recirculation. This gas recirculation is notrequired at all times. Therefore the duct that conveys the recirculatedgases to the furnace does not have these gases passing through it at alltimes during operation of the steam generator. Since this duct leadsdirectly into the furnace a serious problem exists with regard tomaintaining a tight connection of the duct with the wall of the furnaceand of preventing excessive thermal stresses from developing in thisduct due to large temperature variations of the duct wall. This is sobecause when combustion gases are conveyed through the duct the ductwall will generally be at the same temperature as these gases while whenthese gases are not conveyed through the duct the end of the ductconnected with the furnace may be heated to a very high temperaturebecause of the intense heat in the furnace and due to possible eddycurrent of a small amount of hot furnace gases in the duct work adjacentto the furnace. While this problem is encountered with all types offiring it is particularly acute with a so-called pressure fired furnacewherein the pressure in the furnace is superatmospheric and it istherefore essential that the furnace be absolutely tight.

It is an object of this invention to provide an improved gasrecirculation organization and system for a furnace.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the results desiredas hereinafter more particularly set forth in the following detaileddescription of an illustrative embodiment, said embodiment being shownby the accompanying drawing wherein:

FIGURE 1 is a side view, somewhat diagrammatic and partially in sectionshowing a steam generator within which is embodied the presentinvention;

FIGURE 2 is a top view of the gas recirculation duct arrangement and istaken generally from line 2-2 of FIGURE 1;

FIGURE 3 is a vertical sectional view taken along line 3-3 of FIGURE 2and shows the detailed construction of the end of the duct connectedwith the furnace at an enlarged scale.

Referring now to the drawings, wherein like reference characters areused throughout to designate like elements, the illustrative and thepreferred embodiment of the invention shown therein comprises a pressurefired furnace which is provided with burners 12 that are effective tointroduce fuel and air into the furnace where the fuel is burned.The'Walls of the furnace are lined with steam generating tubes 14 whichare connected at their upper ends with the steamand water drum 16 and attheir lower ends with header 1-3. Downcomers 20 extend downwardly fromdrum 16 and are also connected with header 18 with a circulation circuitthus being formed with the water flowing downwardly through icedowncomers 20 and up through steam generating tubes 14. The combustiongases generated by burning fuel in furnace 10 pass upwardly through thefurnace and through gas pass 22 with the gases then being reverselydirected and flowing downwardly through gas pass 24 and outthrough maingas duct 26 which leads to air heater 2 8. Located within gas passes 22and 24 are the high temperature superheater sections 30 and lowtemperature superheater sections 32, respectively, which are connectedwith the steam and Water drum in a conventional manner to super-heat thesteam as desired. Also located in gas pass 24 is a conventionaleconomizer 33.

Hot air for supporting combustion of fuel within furnace 10 is directedto burners 12 through duct 34 which extends from air heater 28 as shown.

As was previously mentioned, there is a tendency in a steam generatorsuch as that illustrated in the drawing for the steam temperature, i.e.,the superheat temperature leaving high temperature superheater 30, todrop with or as the load decreases on the steam generator. The generatoris designed to produce its designed steam temperature at maximum loadand therefore if no control means were provided the steam temperaturewould decrease as the load is decreased below this maximum value.However, it is generally essential that the steam temperature bemaintained substantially constant over a wide load range and thereforecontrol means must be provided to effect this result. This control maybe achieved in a number of ways, and in the illustrative organization itis achieved by two well known means, one of which is by tilting burners12 and theother by gas recirculation. The burners 112 are arrangedgenerally in the corners of the steam generator so that the fuel and airis directed tangentially to a vertical cylinder in the furnace and arevertically tiltable so that the zone of combustion may be movedvertically within the furnace in a manner to maintain the steamtemperature constant over a substantial load range. In addition to thiscontrol, gas recirculation is provided with gas recirculation duct 36extending from main gas duct 26 and being connected to the lower end ofthe furnace for directing hot combustion gases thereinto. Connected intothe duct 36 is the recirculating fan 38 which is effective to forcecombustion gases into the furnace. With these two control means controlof the superheat temperature over a very wide load range may be had, asfor example, with a 575,000 lbs. per hour steam generator, steamtemperature control may be efiected throughout a load range of 180,000lbs. per hour to 575,000 lbs. per hour with tilting burners beingeffective to control the steam temperature throughout a load range of300,000 lbs. per hour to 575,000 lbs. per hour and gas recirculatingeifecting the control through the range of 180,000 lbs. per hour to300,000 lbs. per hour.

The gas recirculation control will not be required at all operatingloads and in order to prevent the reverse flow of hot gases from thefurnace through the gas recirculation duct dampers 40 and 42 areprovided with damper 40 being immediatelyupstream of fan 38 and damper42 being adjacent the inner end of the-duct which is connected to thefurnace with these dampers preventing flow through recirculation duct 36when in their closed position. The inner end of this recirculation ductwhich is connected with the furnace must be joined to the furnace in agas tight manner-with this being particularly important in pressurefired units such as the one disclosed. 'The inner end of the duct istherefore welded to the steam generating tubes 14 that extend along thefurnace walls with these tubes being suitably separated at the ductopening to permit the introduction of recirculated gases into thefurnace through the duct and with the connection of the duct with thefurnace wall being gas tight.

In order to prevent the inner end of the duct from overheating duringperiods. when combustion gases are not flowing through the duct and inorder to prevent the duct wall from being subjected to severetemperature differentials as would occur from intermittent flow ofcombustion gases through the duct, the inner end of the duct is doublewalled as best indicated in FIG. 3 with there being an inner wall 4-4spaced somewhat inwardly of outer wall 46 to provide a relatively narrowpassage 48 entirely around the inner end of the duct and with spacers 50maintaining these walls in properly spaced relation. The passage 48extends from a location immediately upstream of dampers 42 to theextreme inner end of the duct that is connected with the furnace wall.Also provided in the inner end of the, gas recirculation duct 36 areseveral directing vanes 52 which are also double walled as indicated inFIG. 2 providing a narrow passage, with these vanes also extending froma location immediately upstream of dampers 42 to the inner end of theduct 36. Hot air is directed to the duct 36 between dampers 40 and 42when these dampers are in their closed position and for this purpose airduct 54 extends from duct 34 adjacent the outlet of air heater 28 intoduct 36 at a location between these dampers with dampers 56 beingprovided in this air duct for controlling flow through it to gasrecirculation duct 36. This air admitted to duct 36 through duct 54 isquite close to the temperature of the recirculated gases being somewhatlower in temperature than the recirculated gases since it is heated bythese gases and this air flows through the narrow passage 48 formed bythe double walls at the inner end of the gas recirculation duct and alsothrough the passages formed by the double walled directing vanes 52thereby keeping or maintaining the temperature of the duct and thesevanes quite close to that which prevails when gas is being recirculatedthrough the duct. It is a characteristic of these steam generators thatthe temperature of the air leaving the air heater and the temperature ofthe combustion gases directed through the air heater will. be close tothe saturation temperature of the generator which is the temperature ofthe steam generating tubes that line the furnace. This has the advantageof maintaining the duct wall which is connected to the furnace wall atgenerally the same temperature as the furnace wall so that theconnection is not unduly stressed and accordingly will remain gas tight.

Thus withthe present invention when recirculated combustion gases arenot directed through the recirculation duct 36, as is the case atmaximum load and may also be the case throughout a substantial portionof the upper load range when a control in addition to gasrecirculationcontrol is provided, hot air will be introduced into duct36 by opening the damper 56 in air duct 54, which damper isof courseclosed when combustion gases are being recirculated. This introductionof hot air maintains the temperature of the inner end of the duct wallat generally the same temperature that the recirculation combustiongases would maintain it and also at generally the same temperature asthe steam generating tubes that line the furnace wall and to which theduct is connected. Thus the inner end of the recirculation duct is notheated to the extremely high temperature to which it would otherwise besubjected during periods of no gas recirculation and accordinglyoverheating and overstressing of this inner end of the duct iseliminated.

While I have illustrated and described a preferred embodiment of mynovel organization it is to be understood that such is merelyillustrative and not restrictive and that variations and modificationsmaybe made therein withoutdeparting. from the spirit and scope of theinvention. I therefore do not wish, to be limited to the precisedetailsset forth. but desire to avail myself of, such cha nges as fallwithin the purview of my invention.

What is claimed is:

1. In combination a furnace into which fuel is fired and burned, a ductconnected to the furnace for introducing hot combustion gases thereintosaid duct having its inlet connected to receive combustion gasesgenerated in the furnace so as to reintroduce them back into thefurnace, damper means in said duct adjacent the end: connected to thefurnace, said end of the duct connected with the furnace being doublewalled from a location immediately upstream of said damper means withthe walls being spaced to provide a narrow passage, said passage leadingfrom the duct interior upstream of said damper means into the furnace,with the damper means operating on the inner passage of the doublewalled duct only, and means for introducing a cooling gas into said.duct upstream of said damper means when. said combustion gases are notpassing there through.

2. The organization of claim 1 wherein said end of the duct is providedwith directing vanes that extendfrom a location immediately upstream ofsaid dampers toward, the duct extremity connected with the furnace andwhichv are also doubled walled providing a narrow passage between saidwalls leading from said location into the furnace, with this passagebeing free of the damper means.

3. In combination a furnace into which fuel is fired and burned, a ductconnected to the furnace for introducing hot combustion gases thereintosaid duct having its inlet connected to receive combustion gasesgenerated in the furnace so as to reintroduce them backv intothefurnace, damper means in said duct adjacent the end. connected to thefurnace, said end of the duct connected with the furnace being doublewalled from a location. immediately upstream of said damper means withthe walls. being spaced to provide a narrow passage leading from theduct interior upstream of said damper means into the furnace, with thedamper means operating on. the inner passage of the double walled ductonly, means, for introducing hot combustion gases through the duct intothe furnace, means operative to control such introduction, a source ofhot air at a temperature generally corresponding with that of thecombustion gas, and means. for controllably introducing this air intosaid duct when the combustion gases are not introduced thereinto.

4. In an organization of the type described the combina-.

tion of a furnace having heat exchange tubes lining. its walls, anairheater, duct means conveying combustion gases generated in saidfurnace to said air heater as the heating medium therefor, arecirculation duct connected at one end to said duct means and havingits other end supported from the tubes lining the furnace wall with,

this other end being disposed so as to discharge combustion gases intothe furnace, a fan in said recirculation duct for forcing the gasestherethrough into. the; furnace, and adjustable damper means upstream ofsaid fan and another adjustable damper means downstream ofsaid fan insaid recirculation duct with; the latter damper means spaced somewhatfrom said other end' of said recirculation duct, said other end of saidrecirculation duct being double Walled from a location upstream of thesecond mentioned damper means-with said double walls being spacedforming a narrow peripheral passage communicating at one end with theduct. interior upstream of the second mentioned damper and with thefurnace at the other end and with said other adjustable damper meansoperating on the inner passage of the double walled duct only, means forintroducing hot air. from the air heater into said recirculation ductinterme-v diate said dampers, and adjustable means to control suchintroduction of air.

5. The organization of claim 4 including gas directing vanes in saidother end of said recirculation duct extending from a locationimmediately upstream of the second mentioned damper generally to the endof the duct with said vanes being doubled walled and with double walls 6being spaced to form a narrow passage open at both ends with thispassage being free of the damper means.

6. In an organization of "the type described the combination of afurnace in which fuel is burned, an air heater connected to receive thecombustion gases generated in the furnace as the heating medium for theair heater, means for introducing a portion of the combustion gasesleaving the furnace back into the furnace including a duct connected atone end to the furnace wall, means for controlling the flow ofcombustion gases through said duct including damper means in said ductspaced a short distance from said one end, means for controllablyintroducing air from said air heater into said duct upstream of saiddamper means, said one end of said duct from a location immediatelyupstream of said damper means to said one end of the duct having anouter wall and an inner wall complementary with the outer wall andmaintained in spaced relation therewith so as to form a passage betweenthe walls that communicate with the duct interior upstream of the dampermeans and with the furnace interior, and with the damper operating onthe inner passage of the double walled duct only.

7. In combination a furnace in which fuel is burned, means forintroducing a portion of the combustion gases exiting the furnace backinto the furnace including a duct connected at one end to the furnaceand a fan in said duct to force the gases back into the furnace,adjustable damper means intermediate said fan and the end of the ductconnected with the furnace to prevent reverse flow of combustion gasesthrough the duct when the recirculation of gases is not desired, the endof the duct connected with the furnace wall being double walled from alocation immediately upstream of the damper means to the end connectedwith the furnace thereby forming a narrow annular passage about theinner passage of the duct With this annular passage leading from theduct interior upstream of the damper means to the furnace and with thisnarrow annular passage communicating with the furnace and the ductinterior upstream of the damper means, the damper means operating on theinner passage of the double walled duct portion only and means forcontrollably supplying a heated gas between the fan and the dampermeans.

References Cited in the file of this patent UNITED STATES PATENTS616,455 Day Dec. 27, 1898 818,256 Kemp Apr. 17, 1906 1,829,996 LysholmNov. 3, 1931 2,737,931 Koch Mar. 13, 1956 2,875,736 Stallkamp Mar. 3,1959 FOREIGN PATENTS 1,135,897 France Dec. 12, 1956

